Sewage effluent is typically treated in several stages. Larger suspended solids are removed in the first stage. Secondary sewage treatment, which typically involves microbial degradation that mineralizes compounds containing nutrients, thus making them soluble, and other unit processes, removes suspended solids, decreases oxygen demand and disinfects sewage effluent. Secondary sewage treatment generally does not remove nutrients such as nitrogen and phosphorus.
The treatment of secondary sewage containing nutrients with a tertiary process is being mandated in many areas. The presence of nitrogen and phosphorus in secondary sewage can be significantly detrimental, leading to increased chlorine demand, conversion of ammonia into nitrates, and stimulation of algae resulting in low dissolved oxygen concentrations and eutrophication of receiving waters.
The usefulness of prior nutrient removal processes is limited. Each process typically removes only one form of one element. For example, only oxidized forms of nitrogen are removed by denitrification. Ammonia can be stripped by aeration, but only if the wastewater pH is carefully controlled. In addition, many processes result in the need for further treatment. An additional step is required for denitrification of sewage having oxidized forms of nitrogen by methanol addition, for example, and the use of metals to precipitate phosphorus requires an additional pH control step.
There are various biological sewage treatment methods. Biological treatment methods employ microorganisms to degrade pollutants. U.S. Pat. Nos. 5,057,221 to Bryant, et al; 4,877,736 to Flierman; and 4,713,343 to Wilson, et al. describe the use of heterotrophic and methylotrophic organisms, including methanotrophs, to dehalogenate and degrade halogenated organic compounds in wastewater. U.S. Pat. No. 4,954,258 to Little similarly describes the methanotrophic degradation of halogenated aliphatic hydrocarbons, using an alkanol rather than an alkane as a carbon source for the organisms.
U.S. Pat. Nos. 4,959,315 and 5,071,755 to Nelson, et al. also describe the biodegradation of halogenated aliphatic compounds, and specifically describe the use of aromatic compounds such as substituted benzenes to induce degradation. In U.S. Pat. No. 4,925,802 to Nelson et al., the biodegrading activity of selected microorganisms is induced using nontoxic, nongaseous substances such as aromatic amino acids.
Other biological methods for degrading halogenated organic compounds employ various other specific organisms and food sources. For example, the method described in U.S. Pat. No. 5,024,949 to Hegeman, et al. uses certain Pseudomonas bacteria to degrade trichloroethylene. The bacteria employed by Hegeman is capable of sustained growth solely on a hydrocarbon mixture containing a branched chain alkyl-substituted aromatic hydrocarbon. Genetically engineered microorganisms have also been used to degrade. trichloroethylene, as described in U.S. Pat. No. 5,079,166 to Winter, et al. A method for degrading halogenated hydrocarbons utilizing an ammonia-oxidizing bacterium is described in U.S. Pat. No. 5,055,193 to Hooper.
A combination of chemical and biological methods for reducing B.O.D., T.S.S., coliform bacteria, nitrogen and phosphorus to acceptable levels is described in U.S. Pat. No. 4,826,601 to Spratt, et al. A series of treatment steps and cells are employed. Phosphorus is removed by adding alum to combine with soluble phosphate. Nitrogen is removed in a two-step process. Ammonia is oxidized to form nitrates, which are subsequently removed by aerobic bacteria. Spratt is representative of many of the disadvantages of prior nutrient removal processes, in that separate processes are employed to remove nitrogen and phosphorus, and nitrogen removal requires separate oxidation and biological steps.
U.S. Pat. No. 4,721,585 to Santolini, et al., discloses a biological sewage purification method aimed at reducing the nitrogen and phosphorus content of wastewater. In Santolini, raw cellulosic matter is added to fouled water under aerobic conditions, in connection with either secondary or tertiary treatment, to assimilate nitrogen and phosphorus. No quantification of the reduction in nitrogen or phosphorus levels by Santolini is disclosed, however. Also, even under the most favorable conditions disclosed in Santolini, less than ten percent (10%) of the nutrients in sewage would be removed if cellulose were added in the proportions disclosed.
There are several other problems associated with the treatment of wastewater using cellulose devouring bacteria. The addition of cellulose to sewage as described in Santolini would dramatically increase the amount of sludge produced in the treatment process. Already, with conventional treatment systems, approximately one-half ton of dry sludge is produced for every million gallons of domestic sewage produced (or per 10,000 people). Use of a biodegradable waste cellulose source would increase the total dry tonnage of sludge to greater than three (3) tons per million gallons, and would, therefore, be prohibitively expensive.
In addition, the use of solid waste as a cellulose source, as described in Santolini, would, in some respects, pollute the water being treated by introducing or increasing the concentration of toxic substances carried by the refuse. Other sources of cellulose, such as straw or corn stalks, or wood scraps or shavings, would themselves add nutrients to the system as they decomposed.
The present invention is directed to a nutrient removal method which overcomes the problems associated with prior water treatment methods.